The present invention relates to an oil ring for use in an internal combustion engine, and more particularly, to a combined oil ring including side rails made of steel and a spacer expander made of steel.
The oil ring of the invention includes, as shown in the FIGURE, two side rails 1, supported by a space expander 3 in the axial and radial directions thereof, in which an outer peripheral surface 11 of the side rail 1 is in sliding contact with an inner peripheral surface 51 of a cylinder, and an inner peripheral surface 13 of the side rail 1 is in contact with a projection 31 of the spacer expander 3.
This type of oil ring is reduced in width by using side rails having a width of 1.0 mm or less and, therefore, it has good flexibility in the radial direction. Thus, the oil ring is improved in follow-up properties with respect to an inner peripheral surface of the cylinder, and has an excellent sealing function. In order to achieve such a reduction in the width of the oil ring, it is inevitably necessary to use steel ribbons.
The use of such steel wires in a piston ring application, however, gives rise to various problems because of their mechanical properties, and it has thus been desired to overcome these problems. The most serious problem is that steel material suffers from a so-called thermal deformation phenomenon under high temperature conditions such as in a diesel engine, and loses its self-tension. Another serious problem is that, under high load conditions such as in a diesel engine, the spacer expander and side rails are seriously worn out and it is thus necessary to increase their wear resistance.
In order to overcome the above-described problems, in particular, to increase the wear resistance, surface coatings exemplified by hard chromium plating and heat treatment exemplified by a low temperature gas nitriding treatment have been conventionally applied (see, for example, Japanese Patent Publication No. 16502/60, Japanese Utility Model Publication No. 241/69 and U.S. Pat. No. 2,390,417.
In accordance with such surface treatments, however, satisfactory results cannot always be obtained, and it is especially necessary to chose the surface treatment depending on the function of the oil ring. Although the outer peripheral surface of the side rails are most required to have good wear resistance because they are in sliding contact with the inner peripheral surface of a cylinder, if the outer peripheral surface is made of a material containing a large amount of high hardness carbide, the spacer expander is worn out by the peripheral surface of the side rail. On the other hand, if the spacer expander is made of high hardness material, press molding of the spacer expander becomes difficult. In the case of the above-described chromium plating, if a chromium-plated oil ring is used in combination with a chromium-plated cylinder, they are worn out by the contact therebetween and, furthermore, corrosive wear occurs in diesel and a high-octane gasoline engines. Thus, chromium plating is limited in application.
An oil ring subjected to a heat treatment such as a low temperature gas nitriding treatment suffers from the disadvantage that the parent material is softened simultaneously with the hardening of the oil ring surface and, therefore, such heat treatments cannot always increase wear resistance to a satisfactory level although they prevent thermal fatigue. Therefore, even when the side rails and spacer expander are subjected to a low temperature gas nitriding treatment, either the side rails or the spacer expander is abnormally worn out, depending on the type of parent material used for the spacer expander and side rails, and it is thus very difficult to obtain the optimum combination.
The side rail and the spacer expander are relatively slidable with each other. Particularly, the side rail must provide specific wear-resistivity, while the spacer expander must provide sufficient mechanical strength capable of withstanding plastic deformation and pressing deformation for its shaping. Therefore, it would be rather difficult to provide optimum combination therebetween for pro ducing combined oil ring.
According to a piston ring described in U.S. Pat. No. 2,390,417, since the piston ring includes low carbon and chromium content (C:0.2 to 0.27 ), effect attendant to chromium carbide may not be obtainable, and therefore, sufficient wear resistivity may not be obtainable.
Cast iron contains high carbon and chromium content as described in Japanese patent publication No. 53-17966/1978. In the cast iron, graphite exists to provide surface lubrication properties. However, graphite does not provide sufficient mechanical strength, and therefore, such may not be applied to the combined oil ring.